In terms of rules, Umaro is right. There is no reason not to use the Assault Cannon really.
In terms of thematics and sense, the sniper rifle is the obvious choice. It's also not a major downgrade in power.
In terms of comedy, threatening gangers with an elephant gun is where its at.
This.
Sometimes doing what is numerically superior isn't in line with your character concept.
My Merc, for example, has the Firearms Skill Group. Is it the most
efficient use of his BP during CharGen? No...but it fit the character concept.
This gives him the ability to use Sniper Rifles without the Heavy Weapons skill that an Assault Cannon would require. He's not a
sniper, per se, but he can snipe. Or pick up a shotgun. Yeah, he doesn't own those...he carries a highly modified (read: Concealable) Assault Rifle and a few pistols...but he can use them if the situation arises.
Is he as efficient as if I had spent those points on Automatics and Pistols? Firearms Skill Group cost 40BP, whereas with Automatics 4 (AR +2) and Pistols 4 (Heavy Pistol +2) I would have been better with my weapons and saved 4 BP. I can see an argument that would be the best decision...but, again, based on his backstory, he should be able to use whatever firearms are on hand. *Shrug*
I usually am an advocate for specializing your character concept, and efficient use of your points - but "Style over Substance" is also a guiding principle, when it comes to all things Cyberpunk.
-Jn-
Ifriti Sophist